Prion Diseases
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Direct answer
Prions are misfolded proteins that replicate without any nucleic acid: the normal cellular prion protein (PrPc, an alpha-helix-rich membrane glycoprotein encoded by the PRNP gene on chromosome 20) converts into the beta-sheet-rich, protease-resistant scrapie form (PrPsc), which then templates further conversion and accumulates as amyloid plaques in a spongiform brain. Human disease spans sporadic Creutzfeldt-Jakob disease (commonest: rapid dementia, myoclonus, periodic sharp-wave complexes on electroencephalography), familial forms (genetic CJD, Gerstmann-Straussler-Scheinker, fatal familial insomnia), infectious transmission (kuru; iatrogenic CJD from dura mater grafts, cadaveric growth hormone, contaminated instruments) and variant CJD from bovine spongiform encephalopathy-contaminated beef. Because no genome exists, radiation, nucleases and formalin accomplish nothing, and sterilisation demands autoclaving at 134°C for 18 minutes with sodium hydroxide.
What you must remember
- Molecular biology: PrPc is a normal GPI-anchored membrane protein; PrPsc differs only in conformation (alpha-helix to beta-sheet) — insoluble, protease-resistant, and self-propagating by templated misfolding.
- Resistance profile: unaffected by ultraviolet and gamma irradiation, nucleases, formalin, alcohol and routine autoclaving; inactivation requires 1 M sodium hydroxide or sodium hypochlorite plus porous-load autoclaving at 134°C for 18 minutes — or incineration.
- Sporadic CJD: the commonest form (about 85%); age over 60, rapidly progressive dementia with myoclonus, cerebellar and visual signs, death within about a year; EEG shows periodic sharp-wave complexes, cerebrospinal fluid 14-3-3 protein elevated, MRI basal ganglia hyperintensity.
- Variant CJD: bovine-to-human transmission from BSE-contaminated beef — younger patients, psychiatric and painful sensory symptoms first, longer course, florid amyloid plaques, pulvinar sign on MRI, and prion detectable in tonsil and appendix; the UK epidemic followed the 1980s-90s BSE outbreak.
- Genetic and acquired forms: PRNP mutations (D178N fatal familial insomnia, P102L Gerstmann-Sträussler-Scheinker); kuru among the Fore of Papua New Guinea; iatrogenic transmission via dura mater, corneal grafts, cadaveric growth hormone and depth electrodes.
- Animal prionoses: scrapie in sheep (the prototype), bovine spongiform encephalopathy ("mad cow disease"), chronic wasting disease in deer — the zoonotic bridge that produced variant CJD.
- Diagnosis: clinical plus EEG, CSF 14-3-3, MRI; RT-QuIC on CSF offers high specificity; definitive diagnosis is neuropathological.
- No therapy exists: management is supportive care, dignity in nursing, and meticulous instrument handling; Indian CJD series are published from centres such as NIMHANS, which maintains national registry interest.
Why sterilisation decides your viva
The whole examination economy of prions flows from one absence: there is no nucleic acid. Every disinfectant that works by damaging DNA or RNA — ultraviolet light, formaldehyde's alkylating action — finds no target, and formalin actually fixes and stabilises the misfolded protein, making tissue less safe, not more. Heat resistance exceeds bacterial spores, hence: steam sterilisation at 134°C for 18 minutes in a porous-load cycle, with 1 M sodium hydroxide or high-concentration hypochlorite; single-use instruments are preferred wherever prion disease enters the differential, and exposed neurosurgical or ophthalmic sets are withdrawn or destroyed — an operational paragraph administrators will ask you to recite.
Now run the clinical case that carries the same logic. A 64-year-old develops diplopia and clumsiness, then over eight weeks profound dementia with startle myoclonus; EEG shows periodic sharp-wave complexes at one per second, CSF 14-3-3 is positive and MRI shows putaminal and caudate hyperintensity. The differential must exclude autoimmune encephalitis before settling on sporadic CJD, and every needle and shaver used en route joins the prion-precaution pathway. Nothing arrests the disease; the microbiologist's contribution is the diagnosis, the instrument policy and the neuropathological confirmation that closes the case.
How the exam frames it
One-mark questions test definitions: smallest infectious agent, a protein with no nucleic acid, the PrPc-to-PrPsc conformational shift. The sporadic-versus-variant comparison is the essay table — age, duration, presenting features, EEG, plaques, tonsil positivity, pulvinar sign — with the florid plaque as the variant's signature. The Nobel garnish: Gajdusek for kuru (1976), Prusiner for the prion (1997). Traps include "14-3-3 is specific" — it rises after stroke and encephalitis too — and "121°C autoclaving kills it", which it does not; the 134°C for 18 minutes figure is the answer key. Indian framing asks about CJD registries and why blood-donor deferral covers United Kingdom residence during the BSE years.
Frequently asked questions
How does PrPsc differ from PrPc?
Only in conformation — PrPc is alpha-helix-rich and soluble, PrPsc is beta-sheet-rich, insoluble, protease-resistant, and templates conversion of further PrPc molecules.
How are prion-contaminated instruments sterilised?
By porous-load autoclaving at 134°C for 18 minutes with sodium hydroxide or hypochlorite pre-treatment, or by incineration; routine 121°C autoclaving and formalin are inadequate.
What is the role of CSF 14-3-3 protein?
A supportive marker of rapid neurodestruction in sporadic CJD — sensitive but not specific, since stroke and viral encephalitis also elevate it.
How does variant CJD differ from sporadic CJD?
Variant CJD affects younger people, begins with psychiatric and painful sensory features, runs longer, shows florid plaques and the MRI pulvinar sign, and follows BSE-contaminated beef exposure.
What was kuru?
A fatal spongiform encephalopathy of the Fore people of Papua New Guinea, transmitted by ritual cannibalism — the prototype of human transmissible spongiform encephalopathy.